Species and Speciation

A species is a group of similar living things that can interbreed (mate and produce fertile offspring) with each other in nature. Different species cannot normally interbreed; if they do, the offspring are usually weak or infertile (like the mule, the sterile child of a horse and a donkey). Speciation is the process by which one species splits into two over many generations. It is one of the most important processes in evolution, because it is how the enormous variety of life on Earth has built up over billions of years.

• Definition (most used)Can interbreedIn nature, producing fertile young
• Species describedapprox. 2 millionOf an estimated 8 to 10 million
• Time to make a new speciesTypically thousands to millions of yearsBut can be much faster
• Famous exampleDarwin's finches13 species from 1 ancestor in approx. 2 million years
• Two main typesAllopatric, sympatricGeographic vs same-area speciation
• New species discovered each yearapprox. 18,000Mostly insects and microbes

What is a species?

The most common definition of a species is the "biological species concept": a species is a group of living things that can interbreed in nature and produce fertile offspring. By this definition, dogs and wolves are the same species (they can interbreed and produce fertile pups), but lions and tigers are different species (a tigon or liger, the offspring of a lion and a tiger, is usually sterile).

The definition works well for animals that reproduce sexually, but it gets complicated in many cases:

• How do you apply it to bacteria that mostly reproduce by splitting in two?
• How do you apply it to fossils, where there is nobody to interbreed with?
• What about ring species, where neighbouring populations can all interbreed but the ends of the ring cannot?
• What about close species that can interbreed (like polar bears and grizzly bears) but mostly do not?

Modern biology uses several different definitions of "species" depending on the situation. The basic idea (a group of similar living things that breed mostly among themselves) is more important than any single strict definition.

How new species are born: speciation

Speciation happens when one population of a species somehow gets split into two groups that stop interbreeding. Over many generations, the two groups gradually drift apart in their features, until they can no longer interbreed even if they meet again. They have become two species.

The most common way this happens is called allopatric speciation: the population gets split by a physical barrier. Common barriers include:

• A new mountain range pushing up between two halves of a population.
• A river changing course.
• An island getting cut off as sea levels rise.
• An ice age splitting habitats into separate refuges.

Once split, the two populations are subject to slightly different selection pressures and different random mutations. Over thousands or millions of generations they drift apart until they are distinct species.

Darwin's finches: speciation in action

The classic example is the Galapagos finches that Charles Darwin studied. Around 2 million years ago, a single species of finch from the South American mainland was blown to the Galapagos Islands. The descendants of those original birds spread to the different islands of the archipelago, and on each island they faced slightly different conditions: different food, different competitors, different climate.

Over thousands of generations, each island's finches gradually evolved to suit their local conditions. Beaks got bigger for crushing nuts, narrower for catching insects, longer for sipping nectar. Eventually populations on different islands could no longer interbreed. Today there are 13 species of Galapagos finch, all descended from that one ancestor 2 million years ago. They are one of the clearest natural examples of speciation we have.

Speciation without barriers

Speciation can also happen without a physical barrier (called sympatric speciation). This is rarer but does happen, especially when:

• A population starts to use a new food source or habitat, splitting into specialists who prefer the new niche and specialists who prefer the old one.
• Mating preferences change so that some individuals only mate with others who share certain features.
• (In plants) chromosome numbers double in a single generation, creating instant reproductive isolation.

A well-studied example is the apple maggot fly in North America, which originally only fed on hawthorn fruits. Sometime in the 1800s a small group switched to feeding on the new (European-introduced) apple trees, and the two populations now mate at different times of year because apples and hawthorns fruit at different times. They are well on their way to becoming separate species without any physical barrier between them.

Fact The fastest speciation ever observed in nature happened in cichlid fish in African lakes. In Lake Victoria, around 500 species of cichlid fish have evolved from a single ancestor in just 15,000 to 100,000 years: a blink of an eye in evolutionary terms. Each cichlid species has its own specific food and mating preferences, allowing many to coexist in the same lake. They are one of the most spectacular examples of rapid speciation in the entire fossil record.

How many species are there?

Scientists have officially described and named about 2 million species of living things. The true number is probably much higher: most estimates put it at 8 to 10 million, and some go as high as 100 million if you include all the bacteria and microbes. New species are being discovered every day, especially in poorly explored places like the rainforests, deep oceans and remote mountain caves.

Did you know? Some species are essentially not defined by sexual reproduction at all. Many bacteria, plants and some animals (like certain whiptail lizards) reproduce asexually, with daughters being genetically identical (or nearly so) to their mothers. For these creatures, the simple "interbreeding" definition of a species does not apply. Scientists use other approaches based on similarity, ecology and genetics.

Deeper dive: ring species, the in-between case

One of the most fascinating examples of speciation in action is a ring species, where a chain of neighbouring populations can all interbreed with their immediate neighbours, but the two ends of the chain (which sit next to each other) cannot. These cases show speciation captured in mid-process.

The classic example is the greenish warbler, a small bird that lives in a ring of forests around the Tibetan Plateau. Starting from a central population, populations of warblers have spread north, west, south and east around the plateau. Each population can interbreed with its immediate neighbours. But the two populations at the north of the ring, which now meet in central Asia, are unable to interbreed despite living right next to each other. They have effectively become two separate species without any clear point at which the split happened.

Other ring species include the Ensatina salamanders of California (which form a ring around the Central Valley) and the herring gull / lesser black-backed gull complex around the North Pole. In each case, the species concept is being stretched to its limit: at what point along the ring does one species end and the next begin? The answer is that there is no clear point; species can shade into each other gradually.

Ring species show that the divide between "one species" and "two species" is often blurry. Evolution does not happen in clean steps; it happens as a gradient. Modern biology accepts that the line between species is sometimes a useful approximation rather than a strict natural fact.

For the engine that drives speciation, see natural selection. For what happens when species fail to keep up, see extinction.